Mode coupler for circular waveguides



Sept. 26, 1961 w. K. KAHN 3,002,163

MODE COUPLER FOR CIRCULAR WAVEGUIDES Filed Jan. 8, 1960 INVENTOR Walc e1: K .Kahn

ATTORNEYS 'pling either of these 3,002,163 MODE (IOUPLER FOR CIRCULAR WAVEGUIDES Walter K. Kahn, Brooklyn, N.Y., assignor to Polytechnic Institute of Brooklyn, Brooklyn, N.Y., a corporation of New York Filed Jan. 8, 1960, Ser. No. 1,343 Claims. (Cl. 333-21) This invention relates to mode couplers for circular Waveguides. In particular, the invention is concerned with an arrangement for coupling the E mode (the TM mode) with the H mode (the TE mode) in circular waveguides. These are the lowest symmetric modes in a circular waveguide.

The mode coupler of this invention may be employed either as a hybrid coupler, or as a mode transducer, as will appear hereinafter.

An object of the invention is to devise an arrangement for coupling the E mode with the H mode and without coupling these modes to any other mode which may be propagated through the guide at the same operating frequency.

A further object of the invention is to devise an arrangement for converting energy from the E mode to the H mode, or vice versa, and in any desired proportion.

Another object is to devise means for modifying the transmission properties of a circular waveguide to reduce the effective size of the guide with respect to the E mode without making any substantial change with respect to the H mode.

The mode coupler of this invention comprises a section of circular waveguide containing two sets of conductive elements mounted within different annular portions of the space within a linear section of the guide. One set of conductive elements mounted within the outer annular portion of the enclosed space is provided to perturb the E mode to make the guide wavelength of this perturbed mode equal to the guide wavelength of the H mode. The second set of conductive elements is mounted within an inner annular portion of the enclosed space and conples together the E and the H modes without coumodes with any other mode which is able to propagate within the waveguide at the operating frequency.

A convenient form for the conductive elements is that of a thin metallic vane, but other forms may be found to serve the same functions.

The first set of vanes comprises elongated vanes arranged longitudinally of the coupling section, each having one edge attached to the inside surface of the waveguide wall and extending radially inward towards the center, the longitudinal vanes being equally spaced angularly about the axis of the guide. The width of the longitudinal vanes radially of the section is only a fraction of the radius of the section, thus leaving a space of circular area at the center of the waveguide which is not occupied by the longitudinal vanes. Within this free central space is arranged, in spaced transverse planes, the second set of thin metallic vanes which function to couple the E mode with the H mode. Each set of transverse vanes at each of the transverse plane locations comprises a number of elongated vanes arranged in an annular array or ring concentric the axis of the guide, each transverse vane being so oriented that its major axis is inclined with respect to a radial line passing through its center by an angle greater than zero and less than 90,

preferably an angle of approximately 45. By providing a sufliciently large number of transverse vanes in each transverse plane, an incident H mode can be restricted to couple only to the E mode, and vice versa, provided the operating frequency is below the cut-off values of the 4, this relation is not necessary and "ice 2. E mode. The preferred form of the transverse vanes is elliptical.

One embodiment of the invention is illustrated in the accompanying drawing in which FIGURE 1 is an end view of a section of circular Waveguide having two sets of conductive elements mounted therein accordingv to the invention, and FIGURE 2 is a longitudinal sectional view of FIGURE 1 taken along the line 22.

Referring to the drawing, the circular conductor forming the waveguide is shown at 3, the metallic vanes arranged longitudinally'of the waveguide are shown at 4. It is preferred that the ends of these vanes be tapered as shown at 4a and 4b. These vanes are made of goodconductor metal having one edge attached to the circular conductor 3 and positioned in radial planes as shown. in FIGURE 1. The number of longitudinal vanes is not critical, but for best results, the number should be eight or more. Longitudinal vanes 4 should be of a length greater than the length L of the coupling section of the Waveguide in, which the transverse vanes 5 are mounted.

The transverse vanes 5 are formed of thin vanes of good-conductor material and are elongated, the preferred form beingelliptical as shown in FIGURE 1. Transverse vanes 5 are arranged in groups, each group forming a circular array or ring concentric with the axis of the waveguide andlocated in a common plane 6-. As shown in FIGURE 1, each group is formed of eight elliptical vanes 5 arranged in a ring or circular array within the central open space surrounded by the vanes 4,

and the vanes 5 are oriented so that the major axis of each vane is at an angle lying between zero and ninety degrees with respect to the radial plane passing through the center of the vane. The angle for optimum results is substantially 45 as shown in FIGURE 1. While the vanes 5 in FIGURE 1 are radially aligned with the vanes the vanes 5 may be positioned opposite the compartments formed between adjacent pairs of vanes 4. Also, there maybe more than eight transverse vanes in each ring.

As shown in FIGURE 2, the coupling section of the waveguide is formed of a plurality of rings of vanes 5 located in planes 6 spaced apart along the length of the waveguide of a distance Llocated intermediate the ends of the longitudinal vanes 4. The distance of separation between adjacent rings of transverse vanes is designated s. The transverse vanes may be supported in any convenient manner, such as by being embedded in a suitable dielectric material, preferably a material of low dielectric constant such as polyfoam or foamed polyethylene.

The longitudinal vanes 4 are made of sheet metal of a thickness as small as possible consistent with the problem of supporting them in proper position, and they have a width radial of the tube 3 designated at W in FIGURE 2.

The vane width W is selected so that the cutoff wavelength A of the perturbed E (TlVIo1) mode in the coupling region L is approximately equal to the cutoff wavelength of the H (TE mode in that region. Of course, the cutoff Wavelength h of the H mode is unaffected by the thin vanes 4 and so remains the same as for ordinary circular waveguides, that is,

M as

where (a) is the guide radius, W the vane width and N thenumber of vanes which should be 8 or larger. As a result The length L of the coupling region depends upon the coupling per unit lengthfof the ellipses (C/s), and the whereas, should the device function as amode transducer i.e., twice the length of the hybrid.

As already pointed out, the longitudinal vanes funct o to ma the u n th. O the Em m e u 10 that of the H mode. Under this condition, the elementary fields coupled from one mode to the other by the, transverse vanes 5 at each coupling plane reenforced constructively so that over an appreciable length of the coupling region substantial power is transferred. Thus, energy in one mode introduced at one end of the coupling region will be progressively transformed into energy of the other mode, and for certain length of the coupling region indicated above, the energy will be equally divided between the two modes and the device Will function as a hybrid coupler. By doubling the length of the coupling region from that of a hybrid coupler, the energy will be substantially completely converted from the initial mode into the other mode and the device then functions as a modetransducer.

I claim:

1. A mode coupler comprising a sectiorr of circular aveg id fir nduc ng means distribu ed hro ghout an annular portion of the space enclosed within said waveguide adjacent the wall thereof and operating to make the cut-off frequency of the E mode equal to that of the H mode, and second conducting means mounted within the central space enclosed by said first conducting means, said second conducting means being distributed 5g, along a linear section of said wave guide and operating to couple the E mode with the H mode.

2. A mode coupler according to claim 1 wherein said first conducting means comprises thin metallic vanes extending longitudinally of the waveguide and positioned with their planes radially of the axis of said Waveguide.

3 A mode coupler according to claim 2 wherein said second conducting means comprises thin metallic vanes arranged in parallel planes extending transversely of'said waveguide.

4. A mode coupler according to claim 1 wherein said second conducting means comprises a set of thin metallic vanes arranged transversely of the waveguide axis, said transverse vanes being mounted in groups located in parallel planes spaced along the length of said waveguide section, and each group of transverse vanes comprising an annular array of elongated vanes surrounding the axis of the section, each elongated vane having its major axis inclined to a radial line passing through the center thereof by an angle of substantially 45.

5, A mode coupler comprising a section of circular waveguide, a set of elongated metallic vanes mounted within said section and extending longitudinally thereof, each vane having one edge attached to the inside surface of the waveguide wall and extending radially inward towards the center, said vanes being equally spaced angularly about the axis of the guide, the width of the longitudinal vanes radially of the sect-ion being only a fraction of the radius of the section, thereby leaving a free space of circular area at the center of the waveguide, a second set of thin metallic vanes arranged with their planes transversely of the waveguide axis, said transverse vanes being arranged in groups located at ditferent transverse planes spaced along the axis of the waveguide, each group of transverse vanes at each of the transverse plane locations comprising a number of elongated vanes arranged in an annular array concentric with the axis of the guide, each transverse vane being so oriented that its major axis is inclined with respect to a radial line passing through its center by a substantial angle greater than zero and less than 96.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,714 Southworth Sept. 13, 1938 2,519,750 Ehlers Aug. 22, 1950 2,691,766 Clapp Oct. 12, 1954 

